Human brain still evolving

Here's some science news that's sure to cause some controversy: a research group led by Bruce Lahn at the University of Chicago is reporting here and here that they've found evidence for strong positive selection in very recent human history for certain genetic variants of two genes involved in determining brain size (one, microcephalin, is said to have been selected for within the last 37,000 years, and the other, ASPM, within the last 5,800). On its own, this is neither surprising nor controversial: natural selection never stops and still acts on human populations today. But here's the kicker:

They report that with microcephalin, a new allele arose about 37,000 years ago, although it could have appeared as early as 60,000 or as late as 14,000 years ago. Some 70 percent or more of people in most European and East Asian populations carry this allele of the gene, as do 100 percent of those in three South American Indian populations, but the allele is much rarer in most sub-Saharan Africans.

With the other gene, ASPM, a new allele emerged some time between 14,100 and 500 years ago, the researchers favoring a mid-way date of 5,800 years. The allele has attained a frequency of about 50 percent in populations of the Middle East and Europe, is less common in East Asia, and found at low frequency in some sub-Saharan Africa peoples.

I'm bracing myself for racist misinterpretation of these results: people will grab on to this to claim that this is scientific proof that black people are genetically determined to be less intelligent than whites, Asians, and others.

The key thing to notice is that these are only two genes out of several that control brain size (and many, many others that control how the brain is wired up and how it changes with learning and memory):

[Lead author] Dr. Lahn said there may be a dozen or so genes that affect the size of the brain, each making a small difference yet one that can be acted on by natural selection. "It's likely that different populations would have a different make-up of these genes, so it may all come out in the wash," he said. In other words, East Asians and Africans probably have other brain enhancing alleles, not yet discovered, that have spread to high frequency in their populations.

And obviously, brain size is not the whole story when it comes to intelligence - so the set of interesting brain genes isn't just the "dozen or so" regulating brain size, but includes those with more subtle effects on the wiring of neural circuits.

What's more, we don't even know if the selected-for allele actually affects brain size compared to the other alleles - it could have some completely different effect that was selected for. (After all, genes don't encode brain size; they encode proteins that regulate, for example, how long neural stem cells keep dividing. These mechanisms happen at the molecular level, and so could have other effects beyond just cell proliferation.) To look at this, you'd have to examine people's actual brain size and compare that to which alleles of ASPM and/or microcephalin they have.

Fortunately, Dr. Lahn appears to be appropriately cautious about interpreting his group's results. Meanwhile, get ready for people to read too much into these results and then start claiming that it's "just political correctness" that makes this controversial and praising Lahn for his "courage in standing up against the liberal academic orthodoxy" or some other such nonsense.